A study in Scotland found the negative correlation between intelligence test scores and number of siblings held true for all social classes. In France, it was "clearly apparent" for children of farmers and manual workers, but "barely discernible" for children of the professional classes.
That the difference in children of large and small families persists in adult life is indicated by the average scores of army recruits on tests of different types in Great Britain. In the tests that measured general, verbal, and special mechanical intelligence, the recruits from small families scored 11 to 16 percent higher than those from families with five or more children, and the difference increased with increasing family size. On the other hand the difference in tests of physical ability was much smaller, only about 5 percent.
Possible Reasons for Greater Intelligence in Children of Small Families. It is likely that the ability to think abstractly, which underlies most kinds of human problem-solving, develops at an earlier age and to a greater degree if children learn the necessary verbal skills either from adults or from siblings considerably older than themselves. The smaller the family size, the easier it will be for children to develop such skills. These concepts receive support from psychological evidence that suggests that a young child's intelligence level can be raised by the environment in which he is brought up, including the cultural stimuli provided by the family, or by an urban setting. A high proportion of persons of outstanding intellectual achievement were either only children or came from families in which there was a large age gap between siblings.
Children in large families may suffer more maternal deprivation because of greater maternal illness and the stress of large numbers of children on the mother. The effects of extreme maternal deprivation are drastic and impressive. They result in lower linguistic skills and I.Q. scores and less success in later life. In one study, 50 percent of children deprived of maternal care were in a state of dazed stupor—apathetic, silent and sad, making no attempt to make contact with others, often suffering from insomnia, prone to infection, and dropping behind other children in development. The effect of extreme maternal deprivation is also well shown by comparing children brought up in institutions with those brought up in foster homes from early infancy. At the age of 3 years the I.Q.'s of the institutionalized children were 28 points lower than those of the children who had been cared for by foster parents.
An important question remains unanswered: Would the parents of large families among the poor have provided better for their children if they had had fewer of them, and would the children, in consequence, have achieved greater physical and intellectual development? Parents who do limit family size may be qualitatively different from those who do not. If the difference exists, it might result in both smaller numbers of children and healthier, more intelligent children in some families than in others. Alternatively, parents who